The present invention relates to a liquid composition for forming a silica-based coating film or, more particularly, to a liquid composition having excellent storage stability and capable of giving a silica-based coating film having uniformity and a relatively large thickness. The invention also relates to a method for forming a silica-based coating film on the surface of various kinds of substrates such as semiconductor silicon wafers by using a liquid coating composition.
As is well known, it is widely practiced to provide a silica-based coating film to the surface of a substrate such as a semiconductor silicon wafer. The known methods for forming such a silica-based coating film are roughly classified into vaporphase deposition methods and coating methods, of which the former methods of vapor-phase deposition are generally preferred hitherto. Vapor-phase deposition, however, has several problems and disadvantages. For example, the method must be performed by using special apparatus with a limitation in respect of the maximum size of the substrate materials to be treated therein. The method is not suitable for the treatment of a large number of substrates with relatively low productivity in mass production. The method is less versatile than the coating method when the desired coating film should be formed of a mixture of organic and inorganic constituents. It is a trend in recent years, accordingly, that the vapor-phase deposition method is replaced with the more simple and convenient coating method, when possible.
A conventional coating liquid which is applied to the surface of a substrate to form a silica-based coating film is a dispersion of a finely divided powder of silica or glass in a suitable solvent containing a polymeric material dissolved therein as a vehicle. Such a coating liquid is practically not usable because of the poor stability of the dispersoid in the liquid medium readily causing settling therein and the silica-based coating film obtained therefrom is poor with respect to uniformity in thickness. With an object to overcome the above mentioned prob-lems, a proposal has been made in Japanese Patent Publications Nos. 52-16488 and 52-20825 to use a reaction product of a halogeno-silane compound with a carboxylic acid and an alcohol in place of the fine powder of silica. Although this method is naturally free from the problems of precipitation or non-uniformity of the coating film because the film-forming ingredient in the coating liquid is not in the form of a dispersion but in the form of a solution, different disadvantages are caused by the use of a halogeno-silane. The film-forming coating solution unavoidably contains a halogen compound such as hydrogen halide as a by-product of the reaction so that the substrate coated with the coating solution followed by a heat treatment may be subject to corrosion due to the halogen compound. In addition, pin holes and cracks are sometimes formed in a silica-based coating film having a relatively large thickness of, for example, 15 to 40 .mu.m formed by using such a coating solution. Therefore, the coating solution of this type is also practically not usable.
In order to solve the above mentioned problem in respect of corrosion of substrates, Japanese Patent Publication No. 57-39658 has proposed a silica-based film-forming coating solution in which the film-forming constituent is a reaction product of a lower alkoxy silane compound with a lower carboxylic acid and an alcohol in the presence of an organic acid-type reaction promotor. It is also taught the silica-based film-forming coating solution may be admixed with an antimony alcoholate compound as a vitrifying agent with an object of increasing the thickness and uniformity of the coating film formed therefrom. The antimony-containing silica-based film-forming coating solution is indeed capable of giving a coating film having outstandingly high uniformity and large thickness as compared with conventional coating solutions without the problem of corrosion in the substrate coated therewith but the coating solution has a defect of extremely low storage stability and readily gells during storage.
As is described above, none of the silica-based film-forming coating compositions of the prior art is satisfactory from the standpoint of practical use so that it is eagerly desired to develop a coating solution having high storage stability and capable of giving a uniform silica-based coating film with sufficiently large thickness.